12 Days of Conifers: How to Measure a Giant Sequoia
Giant sequoia (Sequoiadendron giganteum) is the world’s largest tree by volume. They only grow on the western slope of the Sierra Nevada, and there are few enough of giant sequoia trees that if you put them all together, the area that they cover would be smaller than Sacramento.
It’s one of several tree species that USGS scientists are monitoring to understand the effects of severe drought, wildfires, and bark beetles on Sierra Nevada forests.
One of the most basic but also most important pieces of data to collect about a tree is its diameter. Measure a tree again and again, and you can calculate how fast it is growing over time. Growth is a key measurement for understanding how drought, fire, and beetles affect a tree—these disturbances stress the tree, forcing it to devote more resources to survival and self-defense and fewer to growth.
In the photo below, biological science technicians Teodora Rautu and Eva Lopez are getting in place to measure the diameter of a giant sequoia in a long-term study plot from the Sierra Nevada Forest Dynamics project. All of the trees in these plots are assessed every year, with the diameters measured every 5 years. Most of the trees can be measured by just one person, but some sequoias are so big that it takes teamwork!
Forest ecologists use a special kind of tape measure to measure tree diameter. These tape measures have a small hook at the end to help it hook onto tree bark, and the numbers are adjusted so that even though you measure around the tree (its circumference), the numbers you read back tell you its diameter. Tree diameters are taken at 4.5 feet up the trunk, to avoid the extra wide area at the base of the tree. When the tree is on a slope, like the one pictured here, you measure from the uphill side.
Research by USGS and partners has found that though giant sequoias are very resilient trees, well-adapted to survive fire and other disturbances, they have been hard-hit by recent severe drought and fire, which weaken the trees and may make them susceptible to beetle kill. USGS scientists are working to understand how these disturbances interact to help managers to develop science-based strategies to protect these iconic trees.